Efforts to combat the childhood obesity epidemic are increasingly focused on schools. This is not surprising given that as much as 40 percent of children's daily energy intake occurs at school, suggesting that the school food environment can have a large impact on their overall diet and energy intake. Despite this policy focus, however, the causal effects of the school food environment on children's diet and well-being are not yet well understood because of the inherent difficulties in addressing potential endogeneity with cross-sectional data. In this study, we propose to use longitudinal data and quasi-experimental methods to estimate the causal effect of one important aspect of the school food environment - specifically, competitive food availability (hereafter, CFA), defined as foods and beverages available for sale in schools outside of the school lunch and breakfast programs - on children's dietary behaviors, body weight and other health- and school-related outcomes. Competitive foods are not required to meet federal nutrition standards, but directly compete with nutritionally regulated school meals. They have lower nutritional value compared to school meals- therefore, commonly referred to as junk foods - but are often more attractive to children due to their taste. Their availability in schools might lead children to substitute calories away from nutritionally-regulated school meals and meals brought from home towards competitive foods, leading to changes in overall diet quality, and subsequently, body weight and child well-being. Our study has three specific aims: 1. Describe the availability of competitive foods in the ECLS-K sample during grades 5 and 8 and identify factors that predict their availability. 2. Estimate th effect of CFA on dietary behaviors including school meal participation, in-school junk food purchases, and overall consumption of specific foods and beverages. 3. Estimate the effect of CFA on body mass index (BMI), overweight and obesity as well as health- and school related outcomes such as physical activity, academics and social-behavioral outcomes. The analyses will rely on data from the Early Childhood Longitudinal Study - Kindergarten Cohort (ECLSK), a 9-year panel on a nationally-representative cohort of children. We will primarily examine outcomes from the 5th (n=9,300) and 8th (n=7,400) grade waves, when information on CFA and children's dietary behavior was collected. Given the complexities of addressing the potential endogeneity of school food environment, we will undertake several well-established econometric approaches including value-added models, propensity score weighting, instrumental variables estimation, and child fixed effects models. Our findings will be critical fo states and school systems across the nation that are seeking to inform decisions on competitive food availability. Public Health Relevance: The school food environment can have a large impact on children's overall diet and energy intake; however, its causal effects on children's diet and well-being are not yet well-understood because of the inherent difficulties in addressing potential endogeneity of the school food environment. Our study will provide comprehensive and unique evidence on the likely causal effects of CFA during two critical points in child development for a national sample of children. Our findings will be critical for states and school systems across the nation that are seeking to inform decisions on competitive food availability.